This invention relates to the preparation of titanium dioxide in an aqueous slurry and more particularly an improved slurry suitable for production of paper or paperboard applications having enhanced optical properties.
Certain finishing steps are required to be performed on conventional TiO.sub.2 slurries such as filtering, washing, drying, micronizing and reslurrying. Conventional finishing steps are more expensive and laborious. Conventional slurries are prepared using pigment that has been micronized so the particles are relatively easy to disperse in water. Base TiO.sub.2 as defined hereinbelow is highly aggregated and agglomerated. If the solids concentration is high, the viscosity becomes too high and dry base TiO.sub.2 particles cannot be readily incorporated into the slurry. Heretofore, base TiO.sub.2 has not been used to directly process a slurry at greater than 83% solids concentration. For example, U.S. Pat. No. 5,356,470 describes a slurry process operating at 79-83% solids content and a process for media milling pigment slurries to eliminate or reduce oversize particles with a particle size distribution between 10-44 microns for enhanced smoothness of paper and paperboard applications. No mention is made therein regarding opacity or a uniform, narrow particle size distribution of smaller particles. One important property of TiO.sub.2 is its ability to opacify.
An efficient, economical process that directly produces a slurry with a narrow particle size distribution by operating at a high solids concentration with dry base TiO.sub.2 particles readily incorporated into the slurry is therefore needed. Concomitantly, there is a need to produce an aqueous TiO.sub.2 slurry with-improved optical efficiency. The present invention meets these needs.